ABS Hydraulic Unit Test Procedures
The ABS hydraulic unit features two diagnostic ports that allows it to be tested for internal leaks, which is likely the most common failure type. To test the hydraulic control unit:
1) Remove the passenger side inner fender.
2) Locate and remove the dust caps from the ABS pump test ports (arrows in figure below).
3) Insert a small screw driver, pick, rod, or dowel into the test port until it bottoms out.
4) Depress the brake pedal. If brake pressure bleeds off, continue to depress the pedal until it bottoms out.
5) Check the position of the tool that was inserted into the test port. If it has been pushed out of the port more than ~1/4", the ABS hydraulic unit has an internal leak. Repeat procedures for 2nd test port.
If the ABS pump passes this test, it is likely not the cause of your brake problems. Note that a failed hydraulic unit does not always trigger an ABS light or DTC, thus concerns that do illuminate the ABS light in the instrument cluster are likely an electrical issue; ABS module, wheel speed sensor, ABS fuse, etc. If brake pressure bleeds off when the pedal is depressed but there is no ABS light or related DTC(s), the ABS hydraulic unit is a probable suspect. Unfortunately, diagnosing ABS brake systems can be difficult and time consuming.
ABS Hydraulic Pump Replacement Procedures
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• Disconnect the negative battery cable.
• Drain the master cylinder completely using a small syringe or suction gun.
• Remove the driver side wheel and tire.
• Remove the driver side inner fender (series of small screws and body clips).
• Disconnect the main electrical connector from the ABS module (large connector at front of unit).
Tip - Pull the connector towards the front of the vehicle with moderate pressure while you pull the connector slide lock towards the passenger side. The connector will progressively remove itself as the slide lock is disengaged.
• Attach a small section of rubber hose (1/8" fuel hose works well) to each of the brake bleeder valves at all 4 brake calipers. Place the hose in a suitable container, such as plastic quart paint cups, and open the bleed valves. The brake system may or may not start draining at this point. It may be necessary to remove the remaining wheels and tires and place the vehicle on jack stands, depending on whether or not the bleed valves on the calipers are accessible.
• Remove the brake lines from the hydraulic unit using a flare nut wrench (7/16" and 1/2" sizes required for pictured unit). Plug the lines with vacuum line caps as they are removed to avoid spillage and contamination.
CAUTION - BRAKE FLUID WILL REMOVE PAINT RAPIDLY
• Remove the front retaining bolt from the base of the hydraulic unit that secures it to the frame mount.
• Remove the (2) rear retaining bolts from the base of the hydraulic unit. The hydraulic unit and mount can now be removed from the vehicle.
• Using an air pump, air compressor, or similar tool, use low pressure (< 10 psi) to evacuate the old brake fluid from the (4) lines that attach to each brake caliper. Do not blow out lines that attach the master cylinder. A MityVac that performs pressure and vacuum procedures works extremely well for evacuating the brake lines. Ensure that all the brake bleeder valves are open and that the lines are being drained into an appropriate container. Brake fluid is corrosive and will remove paint rapidly.
• Add 1 to 2 cups of new brake fluid to the master cylinder reservoir.
• Using a suction apparatus (again, MityVac works well) with the proper attachments, pull all brake fluid from the master cylinder reservoir through the outlet line(s). If all fluid that is removed appears dirty, flush the brake master cylinder once more with clean fluid.
• Install the replacement ABS hydraulic unit and attach all brake lines snug.
• Reattach the main ABS module electrical connector.
• Close the bleed valves at each brake caliper.
• Thoroughly bleed the brake system, then check the hydraulic units for leaks before reinstalling the inner fender. See: F-150 brake bleeding procedures
• After the brake system has been bled conventionally, a "service bleed" is typically required as air will remain trapped in the lower half of the HCU (hydraulic control unit). A scantool with special functionality (such as AutoEnginuity, shown) is required to perform this procedure, which activates the various solenoids in the HCU so that all air can be removed from the system. If you do not have access to such equipment, take it to a service shop that can perform the function.
• After performing the service bleed, re-bleed the system using conventional methods to ensure all air is removed.